This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
dawidszczachor, mitchellbrosius, joryvsminecraft, sakine.26, DO, blood-kun, Loiro, Kirito_SAO_Maniac, xetatorr12, Yellow Foxy Lady, ###@@$$$_smiil babba_###@@$$$, eloelo, superpup1254, Evo, azamani0326, MarlonHedgehogX, SQUAD|Timk024, sankirito2002, Night Shadow, ihuertaalv, Stick615, shiy_aiden